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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 14 July 2011 by Ove Hoegh-Guldberg

Dr. Ove Hoegh-Guldberg is a Professor of Marine Studies and Director of the Global Change Institute at the University of Queensland, and one of the foremost experts on the Great Barrier Reef (GBR). This is the third and final part in the series from Dr. Hoegh-Guldberg on the state of the GBR (following up on Part 1 and Part 2).

How does ocean acidification interact with warming sea temperature?

Coral reefs are susceptible to the impacts of ocean acidification (Raven et al. 2005). Ocean acidification arises from increased amounts of CO2 entering the world's oceans. CO2 reacts with water to create a dilute acid (carbonic acid), which interacts with other chemical species present in sea water such as carbonate ions. In the latter case, carbonate ions are increasingly turned into bicarbonate ions. The two changes brought about by ocean acidification, decreasing pH, and carbonate ion concentrations, impact a wide range of marine organisms (Hendriks et al. 2009; Kleypas and Langdon 2006; Kleypas et al. 2006). There is now a large amount of information showing that reef building corals are susceptible to relatively small changes in ocean acidification (Kleypas and Langdon 2006), probably as a result of declining carbonate ion concentrations in the first instance. An important point is that warming sea temperatures and steadily acidifying oceans interact in terms of their impact on reef building corals. That is, the thermal sensitivity of corals appears to increase as oceans become increasingly acidic (Anthony et al. 2008). The full implications of this interaction between climate change stressors has not been fully explored to date.

What evidence do we have that warming and acidification are affecting the Great Barrier Reef?

There is now very strong evidence that the Great Barrier Reef is being affected by the combination of these two stress factors. In a study of 328 long-lived corals from a full range of habitats on the Great Barrier Reef has revealed that calcification rates have fallen by 15% since 1990, which is unprecedented in the 400 years of coral core records examined (De'ath et al. 2009). Similar results have also been found in a number of sites around the world (Tanzil et al. 2009). While it is not possible to attribute this change to either increasing tropical sea temperatures or ocean acidification on their own, this study does reveal that changes to the conditions within the Coral Sea (which is steadily rising in sea temperature and acidity) are driving some exceptional changes to fundamental reef processes like calcification.

Is there a threshold above which coral-dominated ecosystems will disappear?

Other evidence of the importance of these two factors comes from our current understanding of what limits the growth and distribution of coral reefs worldwide. The current distribution of carbonate coral reefs around the world today is associated with concentrations of carbonate ions of 200 µmol per kilogram water or more. In this case, there is a natural gradient towards reduced carbonate ion concentrations and more acidic oceans at higher latitudes. This is primarily due to the fact that cold water can contain a lot more carbon dioxide.

The significance of this threshold for carbonate ions is that these are the concentrations that you get in tropical oceans when carbon dioxide increases above 450 ppm. Given that these levels of carbon dioxide in the atmosphere are likely to be associated with at least a 2°C increase in sea temperature, it appears that coral reefs will largely disappear if atmospheric concentrations of carbon dioxide exceed 450 ppm. The full background to this argument can be found in the following review paper in Science magazine: (Hoegh-Guldberg et al. 2007). In terms of the distribution of waters that will have the right chemistry for carbonate coral reef ecosystems, the following set of diagrams from this paper outlines how these change as atmospheric levels of CO2 increase (number in top left hand side of each panel). The blue water in these diagrams is essentially that required for carbonate reef ecosystems (which are represented by the pink dots).

Figure 1. Changes in ocean chemistry is a function of atmospheric CO2 concentration. See Hoegh-Guldberg et al. (2007) for further details.

The Great Barrier Reef has undergone major transformations in the geological past. During the glacial periods when large amounts of water have been locked up in glaciers and landlocked ice masses, sea levels were over 100 m lower than they are today. This meant that the Great Barrier Reef lagoon was completely exposed and was a wooded grassland not too dissimilar to those found in Queensland today. All of the islands and reef structures that remained after the water retreated were covered with soil and were unrecognisable as coral reefs.

This has prompted some to ask: Why are we so worried about climate change today if the Great Barrier Reef has undergone these types of changes in the past?

The answer to this question is straightforward and comes down to the issue of the rate of change. The changes in temperature and sea level associated with glacial cycles occurred over 10-20,000 years. In this period of time, global temperatures changed by 5-8 degrees Celsius and atmospheric concentrations of carbon dioxide by 100 ppm. If we compare these sets of changes to that going on today, we see that we are undergoing similar set of changes only we are undertaking them in less than 100 years. That essentially means that the rate of change today is about 100-200 times faster than in the past.

Figure 2. Changes in the Great Barrier Reef and Queensland coastline between glacial and interglacial periods. During last glacial period, sea levels were 100 m lower than they are today, exposing a large part of the Great Barrier Reef which subsequently covered by grasslands and forests.

While some changes in the past were relatively rapid, animals and plants like those associated with the Great Barrier Reef had time to shift their geographic distribution as the environments changed. Today, there are clear signs that the speed of change is exceeding the ability of organisms and ecosystems to shift. The Great Barrier Reef provides an instructive example of how fast reef ecosystems need to migrate if they were to keep pace with changing climate. Corals at the northern end are adapted to warmer temperatures (about 3°C) than those in the southern part of the Great Barrier Reef. If corals were to move southward at a rate which matched an increase in sea temperature of 3°C by 2100, they would necessarily have to travel the entire length of the Great Barrier Reef (2500 km) within approximately 100 years. That means that reef ecosystems would have to travel at the rate of 25 km per year. There is very little evidence that complex ecosystems such as the Great Barrier Reef would be able to travel at this impossibly high rate of movement of the environmental conditions necessary for Coral Reef development.

The final point that needs to be made with respect to the question of why we are worried today if coral reefs of undergone big changes in the past is that of timescale. When coral reefs have experience calamities in the past, they have been absent for very long periods (i.e. millions of years). These timescales are well beyond those that make any sense with respect to humans and their dependence on the ecological services provided by coral reefs.

Putting it bluntly, it is irrelevant that reefs have bounced back in geological time when the timescale of importance for the people depend on coral reefs (such as our vibrant tourist industry here in Australia, or the millions of people around the world need them for their food) is on a month to year basis.

Comments

Thank you for this post. The point about rate of change at the end is a crucial one for nearly all ecosystems (including our agricultural ones) and is frequently misunderstood. I think more posts on this topic (why rate of change matters) would be worthwhile.

I'll second Byron's call for posts on rate of change. As this excellent series of articles on the GBR shows, the expected impacts are major, and a key factor is that they are extremely rapid by geological or ecological timeframes.

If this post doesn't scare the crap out of you, then you are just not thinking hard enough. Take this statement:

"....Given that these levels of carbon dioxide in the atmosphere are likely to be associated with at least a 2°C increase in sea temperature, it appears that coral reefs will largely disappear if atmospheric concentrations of carbon dioxide exceed 450 ppm...."

He is saying that coral reefs will largely disappear at the level of climate change that we are hoping and aiming to restrict it to. And what's worse is that we appear to have little to no chance of limiting climate change to 2 degrees or 450 ppm CO2. In other words, there is no hope for coral reefs - they are going to disappear even under the best case scenario.

And if coral reefs disappear, the whole ocean ecosystem will undergo some form of catastrophic collapse. The consequences of that are just too frightening to think about - and it would appear that we have no will or intent to do anything about it.

I can't comment on acidification as I don't know enough yet. But I do take issue with the perceived slow rate of coral migration. We are not dealing with land snails here, the coral and many of its associated organisms have a free floating larval stage which could easy travel 25km in a day on ocean currents.

Given warm clean shallow seawater with suitable substrate the polyps would be in like Errol. Many don't seem to realise how opportunistic nature is, it would put our capitalistic economies to shame for exploiting any suitable niche. Just think of the money that is spent trying to keep the hulls of boats clean.

Take issue all you like DLB, but do you have any evidence that GBR coral reefs can do what you claim? I've just read the above post by one of the world's foremost coral reef scientists and he says otherwise. Remember we are talking about the reef eco-system, not just coral larvae.

In human terms you're suggesting that just because an individual human can move from point A to point B, moving a city from point A to point B is likewise as simple. The reality is somewhat more complex.

@DLB: It seems scarcely credible that anyone reading could have missed the point that the rate of change is of concern. I do hope you pay more careful attention to the OA not OK series so as to remedy your admitted ignorance of ocean acidification.

Many of you seem to have the belief that The Barrier Reef is like a simple organism, so when it gets hot it will try and move away from the stress. The reality is probably more like the "peanut" chemical equation. At this moment there are probably small colonies of high temperature adapted coral just hanging on in the south. If the weather cools these colonies will probably decline, if the climate warms then they will flourish. It's just like the chemical equation, an environmental change will cause a rapid response to those species it favours.

DLB @9, you appear to be suggesting that the Great Barrier Reef will simply migrate to more southerly waters in the event of continued climate change. Nothing could be further from the truth.

One feature which makes the Great Barrier Reef possible is the wide, shallow continental shelf of the coast of Queensland:

As you can see, that wide continental shelf is absent south of Rockhampton, and the most southerly portions of the reef of Gladstone. Consequently, while a fringe reef may be possible in more southerly waters, it will not be a match to the Great Barrier Reef.

However, even the fringe reef is a dubious proposition. A feature of the East Australian coast up to Hervey Bay is the golden beaches, formed by sand carried north up the coast by surface currents and wave action. At the Spit on the Gold Coast, the sand is no longer carried in to the shore, but instead is deposited just of shore forming, first the Spit, then South and North Stradbroke Islands and finally Fraser Island (the later being clearly visible on the map above). The reason for this change of behaviour is that the line of the coast no longer runs North-North-East, but North, so it does not so strongly intercept the current. North of Fraser Island the coast runs North-North-West, carrying it away from the current so the sand is carried into the ocean.

Corals forming south of Fraser Island would intercept the surface current, and face continuous sedimentation by sand. Given that, it is dubious that they would prosper except in isolated in special circumstances (the north side of headlands, for example). So while some reefs and corals could survive the temperature increase by migrating south, they would not survive as a fringing reef, let alone as the equivalent of the great barrier reef.

All of this ignores the effect of ocean acidification which will stress the surviving corals, no matter how far south they "migrate".

Tom C, of course I am aware of the sandy substrate Sth of Gladstone, there won't be much coral development there because of this reason. However 6000 year bp in Moreton Bay there was quite strong development of coral due to warmer temps and a metre higher sea level. Todays Mud Island in Moreton Bay is a former coral reef, it is only muddy because the Brisbane River estuary is now much closer due to lower sea levels and that water temps in the Bay are too cool. Although in some cleaner areas of the Bay like Peel Island, some coral does have a foothold.

My point is not that The Reef will continue a march down the coast, but the coral of the Sth Barrier Reef will quickly be replaced by coral types from the Nth adapted to warmer water. If you can find some research that says the GBR did not exist 6000 years bp when water temps were higher then I may have to reconsider my argument.

Doug M, I base my hypothesis on what I would consider is a normal curve distribution of a species along an environmental gradient. Could be a possible research project for someone.

If you can find some research that says the GBR did not exist 6000 years bp when water temps were higher then I may have to reconsider my argument.

That is not required by what Ove and commenters have said. Indeed, and I throw this out as a suggestion, you could read the part of the post titled: "Corals survived in past climates with warmer temps and higher CO2 than now, so shouldn't GBR survive now?" Just a suggestion you understand.

Part of the abstract says this "Four major coral species categories, including two key species for reef formation in tropical areas, showed poleward range expansions since the 1930s, whereas no species demonstrated southward range shrinkage or local extinction."

They don't give a mechanism but it is probably related to very fluid larval dispersion.

DLB @12, across a range of studies of reef formation on the Great Barrier reef, first reef formation in the Holocene commenced around 6000 years BP, or towards the end of the HCO, although some formed earlier, and one formed at least 9,000 BP. The reason for the late formation of the majority of the reefs is not obvious, and may only be due to delayed sea level rise. On the other hand, I can come across no clear evidence of elevated temperatures on the GBR during the HCO. Current global temperatures are comparable to peak global temperatures during the HCO, and temperatures where significantly warmer in the NH than in the SH at that time.

@14 I think you are missing at least two significant points:

1) My major concern (and I suspect that of many others) is not the survival of individual coral species. Ignoring the effects of Ocean Acidification, some individual species of coral will undoubtedly survive, but they will survive in small refuges like Moreton Bay (possibly) and various headlands and submerged rocks of the northern NSW coast. Each of these habitats will be fairly small, and relatively isolated, and small isolated habitats support only limited biodiversity.

It is not, therefore, the survival of individual coral species that is under threat. Ignoring OA, many will go extinct, but many more will not. Rather it is the loss of the complex ecosystem that has developed on the Great Barrier Reef. (More pragmatic people might also be concerned about the economic catastrophe the loss of the reef would represent to Queensland.)

2) All this ignores OA which will stress corals no matter how far poleward they migrate. Indeed, based on figure 4 above it will stress corals more the further poleward they migrate. Consequently if CO2 levels go much over 500 ppm, even the widespread survival of coral species must be considered seriously at risk.

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